1. Field of the Invention
The invention relates to a heatable glazing or calendering roll comprising a cylindrical hollow body, a bearing journal for each end of the cylindrical hollow body, a displacement body disposed in the cylindrical hollow body and supply and discharge lines for a fluid heat carrier which flows through the annular gap between the displacement body and the cylindrical hollow body.
2. Description of the Prior Art
Such a roll known for example from German Offenlegungsschrift No. 3 014 891 is used in particular for making and processing paper.
The actual roll body, that is the cylindrical hollow body over which in the centre region thereof, except for the two end regions, the web material to be made or processed runs, is made from cast iron or steel, preferably chilled iron or hardened steel.
In the course of time the demands made as regards uniform thickness and the gloss of the papers important for imprintability have continuously increased, and in particular in recent years there has been a great demand for light thin papers. To obtain in these paper the same thickness deviations in percent as in the hitherto usual thicker papers, higher demands were also made of the profile of the rolls. Account has been taken of this partially by improving the geometrical form of the rolls by advances in grinding and polishing techniques so that today in the production of for example 45 g/m.sup.2 paper the tolerances for the diameter of the roll lie within the .mu.m range.
As early as the 60s investigations were made of the influence of the shape changes of glazing or calendering rolls due to axial and radial temperature differences on the roll profile and thus also on the paper profile (cf. the lecture "Improving the paper profile and the gloss by heated glazing and calendering rolls" held by Peter Rothenbacher, Erich Vomhoff and Michael Zaoralek at the main congress of the OZEPA on Oct. 18, 1984 in Klagenfurt). If in accordance with a usual rule of thumb for a temperature change of 1.degree. C. and a reference length of 1000 mm a diameter change of about 10 .mu.m is assumed, a temperature change of 4.degree. C. with a roll having a rated diameter of 710 mm manifests itself in an increase in the diameter of 15 .mu.m. Even by extremely careful polishing work such a deviation cannot be compensated.
These temperature fluctuations and the resulting form changes can also not be kept under control by careful setting of the temperature of the fluid heat carrier, for example water, steam or oil, so that difficulties are always encountered in this respect.
A further problem resides in that cylindrical bodies cast from iron consist in their outer region of soft cast iron and in their inner region of grey cast iron. These two materials combined to form a unitary cylindrical hollow body have different thermal properties so that both the greater thermal expansion at higher temperatures in the inner region compared with the outer colder shell of the soft cast iron and the bimetal effect due to the different thermal expansion of the wear-resistant outer zone compared with the grey cast iron core in the inner region lead to elastic deformations in the edge region of the cylindrical hollow body and thus of the roll. At some distance from the edge region the roll contracts whilst at the camber end itself a widening takes place. Because of this typical form of the change in shape this phenomenon is referred to as "oxbow effect". This oxbow edge effect in heated calendering rolls can be influenced by appropriately formed thermal insulation of the roll edges as known from German Offenlegungsschrift No. 3 140 425.
More exact investigations have however shown that the known features for compensating the oxbow effect are not adequate, i.e. deformations of the cylindrical hollow body still occur, particularly in the edge region, which far exceed the admissible tolerance fluctuations and have corresponding effects on the quality of the web materials made.